(1) Field of the Invention
This invention relates to a reinforced polyphenylene sulfide (hereinafter referred to as "PPS" for brevity) molded board which is excellent in mechanical properties, such as impact resistance, and thermal properties, such as heat distortion temperature and solder resistance, and which provides a very strong adhesion to a metallic foil even without using an adhesive. This invention also relates to a printed circuit board comprising such a reinforced PPS molded board as an insulating substrate, and a process for preparing this printed circuit board with ease and at low cost.
(2) Description of the Prior Art
So-called printed circuit boards comprising an insulating substrate and a conductor composed of a metallic foil such as a copper foil, which is flatly bonded onto the substrate, are used in large quantities for electric circuits in various household electric appliances, electronic computers, communication facilities and various meters and instruments.
As the insulating substrate for a printed circuit board, there has heretofore been used a composite sheet formed by combining a thermosetting resin such as an epoxy resin, a phenol resin or an unsaturated polyester resin with a base sheet composed of paper, glass fibers or synthetic fibers. These composite sheets are prepared by a process wherein a base sheet composed of paper, glass fibers or synthetic fibers is coated or impregnated with a resin solution, generally termed a "varnish", which is formed by dissolving a thermosetting resin in a solvent; the coated or impregnated base sheet is heated in a drier to vaporize the solvent in the varnish and remove the solvent and to effect polymerization of the resin to form a prepreg of the so-called B-stage; a predetermined number of the so-formed prepregs are superposed upon another and compressed under heating to cure the resin. However, in this process, since it is necessary to remove the solvent from the varnish applied to the base sheet, large expenses are required for recovery and treatment of the solvent. Furthermore, the solvent is scattered in the open air and the working environment is drastically polluted. Moreover, a long time is required for curing the resin. Accordingly, this process is disadvantageous from an economical viewpoint.
It is known that a thermoplastic resin such as polyethylene, polytetrafluoroethylene or polyphenylene oxide may also be used as the insulating substrate for a printed circuit board. However, none of these thermoplastic resins can provide printed circuit boards which are satisfactorily balanced among the thermal properties, the mechanical properties, the chemical resistance and the manufacturing costs.
Japanese Patent Laid-open Application (JP-A) No. 85,380/79 proposes a printed circuit board comprising as an insulating substrate a molded board comprising PPS, which is excellent by itself in heat resistance, chemical resistance, electric properties and flame retardancy, and glass fibers (hereinafter referred to as "GF" for brevity) having a length smaller than 3 mm, and a metallic foil bonded to said substrate through an epoxy type adhesive. However, this printed circuit board is poor in mechanical properties and is especially poor in resistance to the soldering operation which is indispensable for processing and attachment of the printed circuit board, that is, the solder resistance. Moreover, the process for preparing this printed circuit board involves various complicated steps such as the steps of mixing PPS with GF, the melt-kneading step, the injection molding step, the adhesive-coating step and the metal foil-bonding step, and the obtained printed circuit board is very expensive.